The present invention relates to an improved device for use in actuating an explosive charge downhole in a well bore. More specifically, the present invention relates to an improved time delay actuating device for use in actuating an explosive charge downhole wherein the amount of time delay may be varied.
Explosive charges are utilized in wellbores to perform various functions, for example, to perforate a well casing to complete or test a formation, or to set a packer or other device downhole. Due to the time and expense involved in these operations and the explosive power of these devices, it is essential that their operation be reliable. The typical wellbore environment poses severe difficulties for the operation of explosive devices downhole, which thus tends to reduce their reliability. For example, extremes of temperature are common which tend to degrade the operation of explosives, and the presence of heavy drilling muds and debris can interfere with a firing apparatus. Impact responsive firing heads can become fouled by debris and particles settling out from the drilling mud.
In some applications, it is not feasible to utilize an impact responsive firing head. In drill stem testing, a zone to be tested is perforated and various downhole parameters such as temperature and pressure are monitored by instruments mounted between the tubing and the firing head. These are non-fullbore opening devices which typically do not permit a detonating bar to pass through to the firing head. In these applications, therefore, pressure responsive firing devices are desired for use.
A complication introduced in the use of pressure responsive firing devices, is that they require the manipulation of pressure in the annulus or the tubing to actuate the firing device. There are, however, numerous applications which call for the maintenance of a relatively low pressure at the time of explosive actuation, such as where it is desired to perforate the casing underbalanced. This requirement may not be compatible, therefore, with the use of pressure responsive firing devices operated by increasing pressure above hydrostatic.
In U.S. Pat. No. 4,614,156 a device is disclosed for actuating an explosive charge downhole in a wellbore. The device comprises first means for initiating a combustive reaction and actuated in response to a first pressure condition in at least a portion of the wellbore and second means for actuating the explosive charge. The device also includes delay means for providing a combustive reaction initiated by the initiating means and continuing for a time delay period providing sufficient time for an operator to alter the first pressure condition to a second pressure condition desired at the time of explosive actuation. The delay means is operative at the end of the time delay period after initation to actuate the explosive charge. Accordingly, it is thus possible to actuate the explosive charge by means of pressure downhole, while having the capability of reducing the pressure to a desired value, for example, a value desired for shooting underbalanced, before the perforating guns are actuated.
The device further comprises means for providing a signal indicating the actuation of the first means in a form adapted to be transmitted to the surface of the wellbore. Accordingly, the operator can be informed that the delay means has been actuated so that he can begin to bleed off pressure in the wellbore, if so desired, prior to actuation of the explosive.
In accordance with a further aspect of the device, the delay means is disposed in a chamber to which it is adapted to release combustion gas as its combustive reaction proceeds. The device further comprises means for venting the combustive gas released by the delay means from the chamber outwardly of the device. Thus, heat and pressure from the delay means is dissipated outside the device as the combustive reaction proceeds. This aids in preventing a build up of temperature and pressure in the chamber which, if not prevented, may cause the time delay to become unpredictable.
A yet further aspect of the device, a method is provided of perforating the casing of a cased borehole at a desired location and at a desired perforating pressure condition within the casing adjacent the desired location. The method comprises the steps of positioning a perforating means adjacent the desired location increasing the pressure within the casing adjacent the desired location from a first condition to a second, initiating pressure condition greater than the first condition and the desired perforating pressure condition, to initiate a time delayed perforator of the casing; and thereafter reducing the pressure within the casing adjacent the desired location from the initiating pressure condition to the desired perforating pressure condition prior to the perforation of the casing.
Although the above device is acceptable in most circumstances, in some instances, it is desirable to be able to vary the amount of time delay before the actuation of the explosive to accomodate a wide variety of well operations and conditions at the surface.